1. Field of the Invention
The present invention relates to a developing device for use in a copying machine, a facsimile device, a printer or the like, and to a process cartridge and an image forming apparatus using the developing device. Particularly, the present invention relates to a developing device that performs development using a two-component developer composed of a toner and a magnetic carrier, and to a process cartridge and an image forming apparatus using the developing device.
2. Description of the Related Art
There has been conventionally known a developing device that uses a two-component developer composed of a magnetic carrier and a toner. Such a developing device using the two-component developer generally deposits the developer on a developer carrier as a so-called magnetic brush, regulates the thickness of the developer layer to a predetermined thickness, and then develops an electrostatic latent image on a latent image carrier such as a photoreceptor. As the developer carrier, for example, the one having a cylindrical developing sleeve, which is disposed rotatably and carries and conveys the two-component developer, and a magnet roll, which is fixedly disposed within the developing sleeve and has a plurality of magnetic poles arranged around the magnet roll, are used.
Incidentally, the distance between the latent image carrier and the developer carrier needs to be reduced in order to faithfully develop the electrostatic latent image held on the latent image carrier. If the distance between the latent image carrier and the developer carrier is reduced, the thickness of the developer layer formed on the developer carrier accordingly needs to be reduced.
As a conventional technique for regulating the thickness of the developer layer on the developer carrier, there has been widely used a method of regulating the thickness of the developer layer by using a developer regulating member against the developer carrier and using a fixed space formed between a leading end of the developer regulating member and the surface of the developing sleeve (referred to as “doctor gap” hereinafter). When using this method, the amount of developer passing through the developer regulating member can be adjusted by controlling the doctor gap, but the doctor gap needs to be set extremely small in order to form a thin layer of the developer. However, if the doctor gap is small, the amount of developer passing through the doctor gap fluctuates more significantly than when the doctor gap is large, and thereby the layer thickness of the developer formed on the developer carrier becomes nonuniform. Moreover, great stress is applied to the developer when the developer passes through the doctor gap, reducing the life of the developer.
In order to solve such problems, Japanese Unexamined Patent Publication No. H5-6103, for example, proposes a developing device in which magnetic poles of the same polarity are adjacently disposed on a magnet roll of a developer carrier, a developer regulating member is provided between the magnetic poles of the same polarity, and the packing density of a developer in the doctor gap is reduced by using repulsive magnetic fields of these magnetic poles so as to reduce the amount of developer passing through the doctor gap.
However, in this conventional developing device, the developer tends to accumulate in the doctor gap due to long hours of operation and, as a result, the rate of uncharged toner increases, causing a problem that an abnormal image with a scumming or the like is generated easily.